Method and apparatus for transmitting and receiving data using multi-user superposition coding in a wireless relay system

ABSTRACT

A method and apparatus for joint unicast using multi-user superposition coding in a wireless relay system are provided. A BS superposition-encodes first and second data messages directed to first and second MSs, respectively, scheduled at a current scheduling instant. The first and second data messages carry first and second information bit streams for the first and second MSs, respectively. The first MS has a relatively good direct link to the BS, and the second MS has a relatively bad direct link to the BS. The superposition-coded data is transmitted to the first MS and an RS connected between the BS and the first and second MSs. The RS receives the superposition-coded data from the BS, extracts the second information bit stream by decoding the superposition-coded data, and transmits a third data message carrying the second information bit stream to the first and second MSs.

PRIORITY

This application claims priority under 35 U.S.C. §119(a) of a Korean Patent Application filed in the Korean Intellectual Property Office on Mar. 16, 2007 and assigned Serial No. 2007-26114, the entire disclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to data transmission and reception in a wireless relay system, and more particularly, to a method and apparatus for joint unicast using Multi-User Superposition (MUS) coding.

2. Description of the Related Art

One active research area of a future generation communication system, such as the 4^(th) Generation (4G) communication system, involves providing a large amount of data with various Quality of Service (QoS) requirements at high rates to users. To enable high-speed communications, the 4G communication system uses cells having very small radii. This means that a conventional centralized wireless network designing scheme is not viable for implementation of the system. In this context, there is a need for a wireless network design that supports distributed control and actively copes with changes in a cell environment such as additional installations of Base Stations (BSs). Hence, a self-configurable wireless network is required which autonomously configures a wireless network without control of a central system in a distributed fashion to provide communication services.

Techniques used for an ad hoc network should be adopted to deploy the self-configurable network in the 4G communication system. A major ad hoc network is a multi-hop relay cellular network realized by employing a multi-hop relay scheme to a cellular network system including fixed BSs. Typically, the BSs are fixed in position in the cellular network. The resulting less flexibility in configuring a wireless network makes it impossible to provide efficient communication services in a wireless environment experiencing fluctuating changes in traffic distribution or the number of required calls.

In the 4G communication system, to avert this problem, the self-configuration wireless network uses a relay scheme in which data is delivered over multiple hops through a plurality of neighboring Mobile Stations (MSs) and fixed Relay Stations (RSs), to thereby enable fast network reconfiguration according to environmental changes and enable efficient operation of the overall wireless network.

The multi-hop relay wireless network offers the benefits of cell coverage expansion and increased system capacity. When the channel status between a fixed BS and an MS is poor, a multi-hop link is established between them via an RS so that a better radio channel is provided to the MS. Efficient communication services can be provided, especially in a shadowing area with a severe shielding effect caused by buildings.

The RS-based wireless relay system needs to transmit data from a BS to an intended MS efficiently on the downlink. There exists a need for a technique for increasing overall communication efficiency, especially when the BS serves a plurality of MSs and an RS assists downlink transmissions for MSs having poor direct links to the BS.

SUMMARY OF THE INVENTION

The present invention has been made to address at least the above problems and/or disadvantages and to provide at least the advantages described below. Accordingly, an aspect of the present invention provides a method and apparatus for joint-unicasting data to multiple users using superposition coding in a wireless relay system.

Another aspect of the present invention provides a method and apparatus for transmitting data sets directed to different users by superposition coding from a BS in a wireless relay system.

A further aspect of the present invention provides a method and apparatus for establishing a multi-hop link between a BS and an MS via an RS in addition to a direct link between them in a wireless relay system.

According to one aspect of the present invention, a method for transmitting data via an RS in a wireless relay system is provided. A BS encodes a first data message and a second data message directed to a first MS and a second MS scheduled at a current scheduling instant by superposition coding. The first data message carries a first information bit stream for the first MS. The second data message carries a second information bit stream for the second MS. The first MS has a relatively good direct link to the BS, and the second MS has a relatively bad direct link to the BS. The superposition-coded data is transmitted to the first MS and an RS connected between the BS and the first and second MSs by the BS. The RS receives the superposition-coded data from the BS, extracts the second information bit stream for the second MS by decoding the superposition-coded data, and transmits a third data message carrying the second information bit stream to the first and second MSs.

According to another aspect of the present invention, a method for receiving data via an RS in a wireless relay system is provided. A first MS receives superposition-coded data including a first data message carrying a first information bit stream for the first MS and a second data message carrying a second information bit stream for the second MS. The first MS has a relatively good direct link to a BS, which is scheduled along with the second MS having a relatively bad direct link to the BS at a current scheduling instant by a BS. A third data message carrying the second information bit stream directed to the second MS is received from an RS connected between the BS and the first and second MSs. The second information bit stream is extracted by decoding the third data message. The first data message is acquired by removing components related to the second information bit stream from the superposition-coded data. The first information bit stream is extracted by decoding the first data message.

According to a further aspect of the present invention, an apparatus for transmitting and receiving data via an RS in a wireless relay system is provided. A BS superposition-encodes a first data message carrying a first information bit stream for a first MS and a second data message carrying a second information bit stream for a second MS having a relatively bad direct link to the BS. The first and second MSs are scheduled at a current scheduling instant. The superposition-coded data is transmitted. An RS connected between the BS and the first and second MSs establishes multi-hop links, receives the superposition-coded data from the BS, extracts the second information bit stream for the second MS by decoding the superposition-coded data, and transmits a third data message carrying the second information bit stream to the first and second MSs. Herein, the first MS receives the superposition-coded data from the BS, receives the third data message from the RS, extracts the second information bit stream by decoding the third data message, acquires the first data message by removing components related to the second information bit stream from the superposition-coded data, and extracts the first information bit stream by decoding the first data message.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the present invention will be more apparent from the following detailed description when taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a diagram illustrating RS-assisted scheduling and transmission in a cellular system according to an embodiment of the present invention;

FIG. 2 is a diagram illustrating a downlink transmission scenario according to an embodiment of the present invention;

FIG. 3 is a flowchart illustrating a joint unicast operation according to an embodiment of the present invention;

FIG. 4 is a diagram illustrating a message flow for an operation among a BS, an RS and MSs, according to an embodiment of the present invention;

FIGS. 5 and 6 are graphs comparing the present invention with conventional methods in communication performance, when equal time resources are allocated to two MSs;

FIGS. 7 and 8 are graphs comparing the present invention with the conventional methods in communication performance, when transmission rates are allocated at a predetermined ratio to two MSs; and

FIGS. 9 and 10 are graphs comparing the present invention with the conventional methods in communication performance, when equal transmission rates are allocated to two MSs.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Preferred embodiments of the present invention are described in detail with reference to the accompanying drawings. It should be noted that similar components are designated by similar reference numerals although they are illustrated in different drawings. Detailed descriptions of constructions or processes known in the art may be omitted to avoid obscuring the subject matter of the present invention.

The embodiments of the present invention efficiently transmit downlink data from a BS to MSs using superposition coding in a wireless relay system having an RS between the BS and the MSs.

The wireless relay system uses a plurality of RSs in order to increase service coverage and system throughput and reduce communication delay. When the BS serves multiple MSs, an RS assists downlink transmissions from the BS for some users, which in general have poor links to the BS. If an MS has a good direct link to the BS, then the RS is not needed for the MS. A good link refers to a link having a Signal-to-Noise Ratio (SNR) that ensures an error rate equal to or below a required threshold when a user receives data. In the case where the direct link of an MS is of intermediate quality, the MS makes an effective choice between the link to the BS and the link to the RS. For such an intermediate case, a relaying method based on superposition coding is used to increase spectral efficiency.

In a multi-user system, transmission resources are usually time, frequencies, or codes. As data is transmitted simultaneously to a plurality of receivers in the same transmission resources or as a plurality of transmitters transmit data simultaneously in the same transmission resources, system capacity can be significantly increased. For this simultaneous information transmission in the same transmission resources, superposition coding is used.

According to the embodiments of the present invention, superposition coding is used for downlink transmission in a wireless relay system using RSs. When a direct link is impossible between a BS and an MS due to factors such as shadowing, the BS and the MS are connected via an RS. A plurality of RSs can be used depending on the number of MSs served by the BS.

FIG. 1 illustrates RS-assisted scheduling and transmission in a cellular system according to an embodiment of the present invention. In the illustrated case of FIG. 1, a BS serves only a single user on a single channel at each scheduling instant. This channel is used in a time-division manner by the BS and an RS.

Referring to FIG. 1, the BS selects a single MS, MS_(k) or MS₁ according to some criteria, for example, maximal throughput, maximal proportional fairness, etc. at each of first and second scheduling instants 102 and 104 and transmits data to the selected MS either directly or with the help of the RS. If the RS needs to assist the downlink transmission, it transmits data to MS_(k) immediately after data transmission from the BS to the RS and MS_(k), for example at the first scheduling instant 102.

The BS selects a pair of MSs, for example MS_(i) and MS_(j) in a certain scheduling epoch and transmits data to them jointly by superposition coding. Notably, the scheduling epoch features simultaneous unicast to both MSs, rather than broadcast to them. That is, each user receives different data from the jointly transmitted data and completely different scheduling algorithms can be used for the two users. Obviously, the Multi-User Superpositioning (MUS) of the present invention is applicable to the case where one or more users are selected per channel. For convenience sake, the case where two users exist is considered herein. Although many classes of scheduling algorithms can be used for the present invention, embodiments of the present invention will be described based on a standard scheduling algorithm.

FIG. 2 illustrates a downlink transmission scenario according to an embodiment of the present invention.

Referring to FIG. 2, a BS 202 supports downlink transmission to both users, MS₁ 206 and MS₂ 208. The link 214 between the BS 202 and an RS 204 has a very high and stable SNR denoted by γ_(R) [dB]. MS₁ has a good direct link 212 to the BS 202, with an SNR equal to γ₀. Although MS₁ is served via the direct link 212, it also has a good direct link 216 to the RS 204 with an SNR of γ₂₁.

MS₂ has a bad direct link 210 to the BS 202 due to shadowing, for example. So it needs to be supported by the RS 204 in order to receive downlink transmissions from the BS 202. The RS 204 has a direct link 218 to MS₂ 208, with an SNR of γ₂₂. For instance, if the SNR of the direct link 218, γ₂₂ is lower than a minimum SNR (i.e. threshold) required for data reception from the BS 202, the direct link 218 is determined to be bad.

Relaying with superposition coding brings an increased spectral efficiency for MS₁ under the condition of Equation (1): γ₂₁≧γ₀   (1)

However, the increase in spectral efficiency is not significant if γ₂₁ is close to γ₀. In an embodiment of the present invention described later, an MUS scheme is proposed which can gain an increase in the spectral efficiency or a wide range of values γ₂₁. The MUS scheme is divided into two steps.

In Step 1, during a time of N symbols, i.e. an N -symbol duration, the BS 202 transmits the following superposition-coded data according to Equation (2): √{square root over (1−α)}X ₁ +√{square root over (α)}X ₂, 0≦α≦1   (2)

where X₁ denotes an N-symbol message carrying D₁ information bits for MS₁ 206, X₂ denotes an N-symbol message carrying D₂ information bits for MS₂ 208, and α is a coefficient appropriately selected between 0 and 1. For the RS 204 to be able to decode both messages, the following restrictions are put to the transmission rates R₁ and R₂ of the messages X₁ and X₂., shown in Equation (3):

$\begin{matrix} {R_{1} \leq {{\log_{2}\left( {1 + \frac{\left( {1 - \alpha} \right)\gamma_{R}}{1 + {\alpha\;\gamma_{R}}}} \right)}\mspace{25mu} R_{2}} \leq {\log_{2}\left( {1 + {\alpha\;\gamma_{R}}} \right)}} & (3) \end{matrix}$

Via the direct link 212 from the BS 202, MS₁ 206 receives the signal as shown in Equation (4): γ₁₁ =h ₀(√{square root over (1−α)}X ₁ +√{square root over (α)}X ₂)+z ₁₁   (4)

where h₀ denotes a channel impulse response representing the channel characteristics between MS₁ 206 and the BS 202 and z₁₁ denotes other signals (e.g. noise) introduced to MS₁ 206. Then MS₁ 206 waits for Step 2, i.e. transmission of the RS 204. On the other hand, having the bad direct link 210 to the BS 202, MS₂ 208 completely ignores the transmission from the BS 202.

In Step 2, during M symbols following to the N symbols of Step 1, i.e. during the next M-symbol duration, the RS 204 transmits a message X₃ carrying the D₂ information bits for MS₂ 208 at a rate R_(s2), according to Equation (5): R _(s2)≦log₂(1+min(γ₂₁,γ₂₂))   (5)

so that both MS₁ 206 and MS₂ 208 are able to receive it from the RS 204 and decode it. For easier notation, as shown in Equation (6): γ₂=min(γ₂₁,γ₂₂)   (6)

To use maximal possible rates in Step 1 and Step 2, the following relationship of Equation (7) should be placed

$\begin{matrix} {{R_{2}N} = {\left. {R_{s\; 2}M}\Leftrightarrow M \right. = {N\frac{\log_{2}\left( {1 + {\alpha\;\gamma_{R}}} \right)}{\log_{2}\left( {1 + \gamma_{2}} \right)}}}} & (7) \end{matrix}$

After the transmission of the RS 204, MS₂ 208 decodes the intended unicast message X of D₂ bits. MS₁ 206 also decodes the message X, creates the D₂ bits intended for MS₂ 208 out of that message X, and reconstructs the message X₂ using the D₂ information bits. Then MS₁ 206 acquires the message X₁ by subtracting the components of the message X₂ from the received signal described as Equation (4) according to the following equation. Thus, MS₁ 206 achieves the intended D₁ information bits by decoding the message X₁., according to Equation (8): γ₁₂=γ₁₁ −h ₀ √{square root over (α)}X ₂ =h ₀√{square root over (1−α)}X ₁ +z ₁₁   (8)

The transmission rate R₁ of the message X₁ satisfies the condition of Equation (9):

$\begin{matrix} {{R_{1} \leq {\log_{2}\left( {1 + {\left( {1 - \alpha} \right)\gamma_{0}}} \right)}},\mspace{25mu}{\gamma_{0} = \frac{{h_{0}}^{2}}{\sigma^{2}}}} & (9) \end{matrix}$

where σ² denotes Gaussian noise power. Equation (3) describes the transmission rates of the messages X₁ and X₂ received in combination at the RS 204 and Equation (9) describes the transmission rate of data received at MS₁ 206 after interference cancellation.

Hence, the total transmission rate of this system can be calculated as shown in Equation (10):

$\begin{matrix} {R_{MUS} = {\frac{N\left( {R_{1} + R_{2}} \right)}{N + M} = \frac{R_{1} + R_{2}}{1 + \frac{R_{2}}{R_{s\; 2}}}}} & (10) \end{matrix}$

FIG. 3 is a flowchart illustrating a joint unicast operation according to an embodiment of the present invention. Steps 302 through 314 can be performed by the BS or by another network node above the BS. For convenience' sake, it will be described that these steps are performed in the BS.

Referring to FIG. 3, the BS acquires given parameters, i.e. the number of users to be served, K and SNRs representing the channel gains of the RS and the BS with respect to the individual users in step 302. K is equal to the number of MSs that intend to receive data within the service area of the BS and the SNRs are obtained from channel state information measured and reported to the BS in measurement reports by the MSs. Also, the BS sets the number of transmission symbols, N, that it will use at each scheduling instant.

In step 304, the BS generates all possible user pairs associated with the K users and selects a first user pair k (k=1). Herein,

$1 \leq k \leq {\frac{K\left( {K - 1} \right)}{2}.}$ Let the MS having the higher SNR of a BS-RS link be denoted by MS₁ and the other MS be denoted by MS₂. Then, the BS checks the SNR between each MS and the RS in step 306. Specifically, the MS₁-RS link has an SNR of γ₂₁ and the MS₂-RS link has an SNR of γ₂₂. The smaller SNR between them is denoted by γ₂. As stated before, the BS-RS SNR is γ_(R).

In step 308, the BS calculates the transmission rates R₁ and R₂ for MS₁ and MS₂ with respect to the given coefficient α in Step 1 according to Equation (11): R ₁=log₂(1+(1−α)γ₀) R₂=log₂(1+αγ_(R))   (11)

Then, the BS calculates overall rates r₁ and r₂ for MS₁ and MS₂ at the scheduling instant by Equation (12). Note that the sum of r₁ and r₂ is equal to R_(MUS) described in Equation (10).

$\begin{matrix} {{r_{1} = \frac{\log_{2}\left( {1 + {\left( {1 - \alpha} \right)\gamma_{0}}} \right)}{1 + \frac{\log_{2}\left( {1 + {\alpha\;\gamma_{R}}} \right)}{\log_{2}\left( {1 + \gamma_{2}} \right)}}}{r_{2} = \frac{\log_{2}\left( {1 + {\alpha\;\gamma_{R}}} \right)}{1 + \frac{\log_{2}\left( {1 + {\alpha\;\gamma_{R}}} \right)}{\log_{2}\left( {1 + \gamma_{2}} \right)}}}} & (12) \end{matrix}$

The BS decides an overall scheduling objective function for the scheduling instant using the overall rates r₁ and r₂ by a known scheduling algorithm in step 310. The overall scheduling objective function depends on the used scheduling algorithm. It outputs a scheduling gain for the input of the overall rates r₁ and r₂ and when needed, other parameters according to the scheduling algorithm. For example, the scheduling algorithm can use proportional fairness measurements as a criterion. Then the BS determines a coefficient α_(k) maximizing the scheduling gain for the user pair k and determines a scheduling gain G_(k) corresponding to the coefficient α_(k).

If the next user pair remains in step 312, the BS selects the next user pair k (k=k+1) and returns to step 306. In the absence of the next user pair, the BS selects a user pair k having a maximal G_(k) and a coefficient α_(k) corresponding to the maximal G_(k) in step 314 and transmits data to the selected user pair in step 316. The specific operation of step 316 is described in FIG. 4.

Referring to FIG. 4, the BS transmits a message X₁ of N symbols carrying information bits for MS₁ and a message X₂ of N symbols carrying information bits for MS₂ using the coefficient α_(k) selected in step 314 of FIG. 3 by superposition coding described in Equation (2) in step 402. The message X₁ includes NR₁=D₁ information bits according to the transmission rate R₁ calculated in step 308 and the message X₂ includes NR₂=D₂ information bits according to the transmission rate R₂ calculated in step 308. The superposition-coded data is delivered to MS₁ having a good direct link to the BS.

In step 404, the RS acquires the two messages X₁ and X₂ by decoding the superposition-coded data and transmits the D₂ information bits extracted from the message X₂ in M transmission symbols at the transmission rate R_(s2) calculated by Equation (5). R_(s2) and M are computed by Equation (13) below. Both MS₁ and MS₂ are able to receive the data from the RS.

$\begin{matrix} {{R_{s\; 2} = {\log_{2}\left( {1 + \gamma_{2}} \right)}}{M = \frac{D_{2}}{\log_{2}\left( {1 + \gamma_{2}} \right)}}} & (13) \end{matrix}$

In step 406, MS₁ receives the superposition-coded data from the BS and the data from the RS, acquires the message X₁ by subtracting the components of the message X₂ from the superposition-coded data referring to the D₂ information bits resulting from decoding the data received from the RS, and then achieves the intended D₁ information bits by decoding the message X₁.

Now a description will be made of the performances of the MUS scheme according to the embodiment of the present invention in many scenarios. In the following description, a conventional scheduling strategy is considered.

FIG. 5 is a graph comparing the present invention with conventional methods in communication performance expressed as normalized rates, when equal time resources are allocated to two MSs.

The two MSs, MS₁ and MS₂, can use N₁ transmission symbols, respectively. The BS transmits information bits in N₁ transmission symbols directly to MS₁ and in N₁ transmission symbols to MS₂ via the RS. Specifically, for a duration of N₁ transmission symbols, the BS transmits the information bits for MS₁ at a rate shown in Equation (14): R _(c1)=log₂(1+γ₀)   (14)

For a duration of N₁ symbols, the BS and the RS transmit the information bits to MS₂ in a multi-hop manner at a rate shown in Equation (15):

$\begin{matrix} {R_{c\; 2} = \frac{{\log_{2}\left( {1 + \gamma_{R}} \right)}{\log_{2}\left( {1 + \gamma_{22}} \right)}}{{\log_{2}\left( {1 + \gamma_{R}} \right)} + {\log_{2}\left( {1 + \gamma_{22}} \right)}}} & (15) \end{matrix}$

For a conventional method without superposition coding, the overall data rate is given by Equation (16):

$\begin{matrix} {R_{conv} = \frac{R_{c\; 1} + R_{c\; 2}}{2}} & (16) \end{matrix}$

When the N₁ symbols are transmitted to MS₁, another conventional method achieves a transmission rate R_(srd2) as shown in Equation (17):

$\begin{matrix} {R_{{srd}\; 2} = \frac{{\log_{2}\left( {1 + \gamma_{R}} \right)}{\log_{2}\left( {1 + \gamma_{22}} \right)}}{{\log_{2}\left( \frac{\gamma_{R}}{\gamma_{0}} \right)} + {\log_{2}\left( {1 + \gamma_{22}} \right)}}} & (17) \end{matrix}$

where srd represents source, relay, and destination.

The present invention uses the MUS scheme for 2N, symbols and its overall transmission rate is determined by using Equation (10). For the MUS scheme, the coefficient α is computed by Equation (18):

$\begin{matrix} {\frac{\left( {1 - \alpha} \right)\gamma_{R}}{1 + {\alpha\;\gamma_{R}}} = {\left. {\left( {1 - \alpha} \right)\gamma_{0}}\Rightarrow\alpha \right. = {\frac{1}{\gamma_{0}} - \frac{1}{\gamma_{R}}}}} & (18) \end{matrix}$

where if it is assumed that γ_(R)>γ₀, α is positive. In this case, substitution of Equation (18) into Equation (13) leads to Equation (19):

$\begin{matrix} \begin{matrix} {{R_{1} = {\log_{2}\left( {\frac{\gamma_{0}}{\gamma_{R}} + \gamma_{0}} \right)}};} & {R_{2} = {\log_{2}\left( \frac{\gamma_{R}}{\gamma_{0}} \right)}} \\ {{{R_{1} + R_{2}} = {\log_{2}\left( {1 + \gamma_{R}} \right)}},} & {R_{s\; 2} = {\log_{2}\left( {1 + \gamma_{2}} \right)}} \end{matrix} & (19) \end{matrix}$

Then the overall rate for the MUS scheme is given by Equation (20):

$\begin{matrix} {R_{MUS} = \frac{\log_{2}\left( {1 + \gamma_{R}} \right)}{1 + \frac{\log_{2}\left( \frac{\gamma_{R}}{\gamma_{0}} \right)}{\log_{2}\left( {1 + \gamma_{2}} \right)}}} & (20) \end{matrix}$

In FIG. 5, two groups of curves are illustrated, one having γ₂₁=γ₂₂=2γ₀ and the other having γ₂₁=γ₂₂=γ_(R)=30 dB. When γ₂₁ and γ₂₂ are fixed, it is noted that the MUS scheme of the present invention with R_(MUS) is superior to the two other conventional methods with R_(conv) and R_(srd2) and the improvement becomes better when γ₀ increases. Also, the increase of the value of γ₂ makes the improvement brought by the MUS more significant. The two curves for R_(MUS) converge at γ₀=30 dB, although the curves have different values γ₂ at this point. This is explained by the fact that when γ₀=γ_(R), the data for MS₂ does not exist i.e. D₂=0.

FIG. 6 is another graph comparing the present invention with the conventional methods in communication performance expressed as normalized rates, when equal time resources are allocated to two MSs and γ₂₁≠γ₂₂. For each of the present invention and the conventional methods, two curves are illustrated for the two cases of γ₂₁=γ₂=2γ₀, γ₂₂=5γ₀ and γ₂₁=2γ₀, γ₂₂=γ₂=0.5*γ₀. As noted from FIG. 6, the transmission rate R_(MUS) of the present invention is beneficial in terms of improving fairness between two users.

FIG. 7 is a graph comparing the present invention with the conventional methods in communication performance expressed as normalized rates, when a transmission rates are allocated at a predetermined ratio to two MSs. Two groups of curves are illustrated, one having γ₂₁=γ₂₂=2γ₀ and the other having γ₂₁=γ₂₂=γ_(R)=30 dB.

The numbers of information bits intended for MS₁ and MS₂ are D₁ and D₂, respectively, and the transmission rates for the MSs should be determined by finding the time that is needed to transfer the data to them. D₁ and D₂ are computed by Equation (21):

$\begin{matrix} {{D_{1} = {N_{1}{\log_{2}\left( {\frac{\gamma_{0}}{\gamma_{R}} + \gamma_{0}} \right)}}};\mspace{14mu}{D_{2} = {N_{1}{\log_{2}\left( \frac{\gamma_{R}}{\gamma_{0}} \right)}}};\mspace{14mu}{\alpha = {\frac{1}{\gamma_{0}} - \frac{1}{\gamma_{R}}}}} & (21) \end{matrix}$

The total symbol duration consumed to send the data is shown in Equation (22):

$\begin{matrix} {{{N_{1} + N_{2}},{where}}{N_{2} = \frac{D_{2}}{\log_{2}\left( {1 + \gamma_{2}} \right)}}} & (22) \end{matrix}$

Then the ratio between the transmission rate R_(MS1) for MS₁ and the transmission rate R_(MS2) for MS₂ is computed by Equation (23):

$\begin{matrix} {\frac{R_{{MS}\; 1}}{R_{{MS}\; 2}} = \frac{\log_{2}\left( {\frac{\gamma_{0}}{\gamma_{G}} + \gamma_{0}} \right)}{\log_{2}\left( \frac{\gamma_{R}}{\gamma_{0}} \right)}} & (23) \end{matrix}$

The BS now determines the numbers of transmission symbols that are needed to transfer the information bits D₁ and D₂ by direct transmission to MS₁ and multi-hop transmission to MS₂. The number of symbols needed for the direct transmission, shown in Equation (24), is M₁ where

$\begin{matrix} {M_{1} = {N_{1}\frac{\log_{2}\left( {\frac{\gamma_{0}}{\gamma_{R}} + \gamma_{0}} \right)}{\log_{2}\left( {1 + \gamma_{0}} \right)}}} & (24) \end{matrix}$

On the other hand, the number of symbols consumed by the multi-hop transmission to send D₂ bits to MS₂ is shown in Equation (25):

$\begin{matrix} {M_{2} = {N_{1}\left( {\frac{\log_{2}\left( \frac{\gamma_{R}}{\gamma_{0}} \right)}{\log_{2}\left( {1 + \gamma_{R}} \right)} + \frac{\log_{2}\left( \frac{\gamma_{R}}{\gamma_{0}} \right)}{\log_{2}\left( {1 + \gamma_{22}} \right)}} \right)}} & (25) \end{matrix}$

FIG. 7 shows that the MUS scheme of the present invention with R_(MUS) is superior to the two other conventional methods with R_(conv′) and R_(srd2′), when the ratio between the transmission rates of the two MSs is preset.

FIG. 8 is another graph comparing the present invention with the conventional methods in communication performance expressed as normalized rates, when a transmission rates are allocated at a predetermined ratio to two MSs and γ₂₁≠γ₂₂. For each of the present invention and the conventional methods, two curves are illustrated for the two cases of γ₂₁=γ₂=2γ₀, γ₂₂=5γ₀ and γ₂₁=2γ₀, γ₂₂=γ₂=0.5*γ₀. As noted from FIG. 8, even when γ₂₁≠γ₂₂, the transmission rate R_(MUS) of the present invention offers much higher gains than in the conventional methods, as far as γ₀ is sufficiently high.

FIG. 9 is a graph comparing the present invention with the conventional methods in communication performance expressed as normalized rates, when equal transmission rates are allocated to two MSs. Two different cases, Case 1 and Case 2 are considered herein. Case 1 satisfies Equation (26):

$\begin{matrix} {\gamma_{0} > \frac{\gamma_{R}}{\sqrt{1 + \gamma_{R}}}} & (26) \end{matrix}$

In this case, the coefficient α is selected according to Equation (27) such that the basic and the superposition-coded stream from the BS have equal rates. This means that, after the transmission of the RS, both MS₁ and MS₂ should be able to receive the same amount of data over the same time.

$\begin{matrix} {{\frac{\left( {1 - \alpha} \right)\gamma_{R}}{1 + {\alpha\;\gamma_{R}}} = {\alpha\;\gamma_{R}}},\mspace{25mu}{\alpha = \frac{\sqrt{1 + \gamma_{R}} - 1}{\gamma_{R}}}} & (27) \end{matrix}$

With such a choice of α, the message X₁ can be decoded by MS₁ after the transmission of the RS, since the following condition is satisfied in Equation (28):

$\begin{matrix} {{{{if}\mspace{14mu}\gamma_{0}} > \frac{\gamma_{R}}{\sqrt{I + \gamma_{R}}}},{{{then}\mspace{14mu}\left( {1 - \alpha} \right)\gamma_{0}} > \frac{\left( {1 - \alpha} \right)\gamma_{R}}{1 + {\alpha\;\gamma_{R}}}}} & (28) \end{matrix}$

As a consequence, the overall rate achieved by the MUS scheme is found to be

$\begin{matrix} {R_{MUS} = {\frac{\log_{2}\left( {1 + \gamma_{R}} \right)}{1 + \frac{\log_{2}\left( {1 + {\alpha\;\gamma_{R}}} \right)}{\log_{2}\left( {1 + \gamma_{2}} \right)}} = \frac{\log_{2}\left( {1 + \gamma_{R}} \right)}{1 + {\frac{1}{2}\frac{\log_{2}\left( {1 + \gamma_{R}} \right)}{\log_{2}\left( {1 + \gamma_{2}} \right)}}}}} & (29) \end{matrix}$

Case 2 satisfies Equation (30):

$\begin{matrix} {\gamma_{0} \leq \frac{\gamma_{R}}{\sqrt{1 + \gamma_{R}}}} & (30) \end{matrix}$

In this case, the coefficient α is selected according to Equation (31): (1−α)γ₀=αγ_(R), α=γ₀/γ₀+γ_(R)   (31)

The amount of data decoded by MS1 after Step 1 and Step 2 is equal to that amount of data that RS decodes for MS₂ in Step 1. After RS forwards that data to MS₂ and MS₁, both MSs extract exactly the same amount of data, thus achieving equal rates after the two steps of the present invention. Under the condition of Equation (30), the RS can easily decode the message X₁ intended for MS₁ received from the BS. As a consequence, the overall rate achieved by the MUS scheme is found in Equation (32) to be R _(MUS)=2 log₂(1+γ₀)/1+log₂(1+αγ_(R))/log₂(1+γ₂)   (32)

In the scenarios of FIG. 9, since the transmission rates R_(MS1) and R_(MS2) of MS1 and MS2 are equal, Equation (33) is satisfied: R _(MS1) =R _(MS2)=1/2R _(MUS)   (33)

Under the condition of Equation (33), the number N₁ of symbols transmittable to MS₁ via the direct link from the BS and the number N₂ of symbols transmittable to MS₂ via the RS are placed in the relationship of Equation (34): N ₁ log₂(1+γ₀)=N ₂ log₂(1+γ_(R))·log₂(1+γ₂₂)/log₂(1+γ₂)+log₂(1+γ_(R))  (34)

Referring to FIG. 9, two groups of curves are illustrated, one having γ₂₁=γ22=2γ₀ and the other having γ₂₁=γ₂₂=γ_(R)=30 dB. As noted from the curves, there are significant regions of γ₀ where the MUS scheme offers higher rates.

FIG. 10 is another graph comparing the present invention with the conventional methods in communication performance expressed as normalized rates, when equal transmission rates are allocated to two MSs. For each of the present invention and the conventional methods, two curves are illustrated for the two cases of γ₂₁=γ₂=2γ₀, γ₂₂=5γ₀ and γ₂₁=2γ₀, γ₂₂=γ₂=0.5*γ₀. As noted from FIG. 10, the MUS scheme of the present invention with the transmission rate R_(MUS) relatively outperforms the conventional methods with the transmission rates R_(conv′) and R_(srd2′).

A description will now be made of exemplary operations based on the MUS scheme according to the embodiment of the present invention. It is assumed that the BS schedules the users according to some criterion i.e. proportional fairness or maximal rate.

In a first operation, the BS decides to serve MS_(i) over a direct link with γ_(0i). Then the BS attempts to find MS_(j) such that Equation (35) is satisfied

$\begin{matrix} {\gamma_{2j} = {\min\limits_{k \neq i}{{\gamma_{2k} - \gamma_{2i}}}}} & (35) \end{matrix}$

After identifying MS_(j), the BS determines whether there is a rate improvement if the MUS scheme is applied for MS_(i) and MS_(j). If the rate improvement can be achieved, the BS transmits data to the two MSs according to the MUS scheme by tuning the coefficient α so as to satisfy a predetermined rate criterion.

In a second operation, the BS decides to serve MS_(i) over a multi-hop link. Then, the BS attempts to find MS_(j) that has γ_(2j) close to γ_(2i), but also a very good link γ₀ _(j). γ_(2i) is the SNR of the multi-hop link to MS_(i) and γ_(2j) is the SNR of the multi-hop link to MS_(j). In case there are several candidates {j}, then the BS can calculate the rates achievable for each user {j}. If there is a user candidate {j} having a maximal rate than a predetermined transmission rate, the BS sends data to the user by using the MUS scheme.

Now a description will be made of the case where there are K users in the system and the system throughput should be maximized. At each scheduling instant, the BS calculates a rate achieved by scheduling a user pair MS_(i) and MS_(j). Here it calculates the overall rates of the MSU scheme and other transmission schemes. Finally, the BS selects a transmission scheme and a pair of users or a single user that maximize the overall rate and transmits data to the selected user pair or single user in the selected transmission scheme.

As is apparent from the above description, the present invention advantageously improves communication quality and increases data throughput by providing a multi-hop link via an RS between a BS and an MS in addition to a good direct link between them, if possible.

While the invention has been shown and described with reference to certain preferred embodiments of the present invention thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the appended claims and their equivalents. 

1. A method for transmitting data via a Relay Station (RS) in a wireless relay system, comprising the steps of: encoding a first data message and a second data message directed to a first Mobile Station (MS) and a second MS scheduled at a current scheduling instant by superposition coding by a Base Station (BS), the first data message carrying a first information bit stream for the first MS, the second data message carrying a second information bit stream for the second MS, the first MS having a relatively good direct link to the BS, and the second MS having a relatively bad direct link to the BS; transmitting the superposition-coded data to the first MS and the RS connected between the BS and the first and second MSs by the BS; receiving the superposition-coded data from the BS; extracting the second information bit stream for the second MS by decoding the superposition-coded data; and transmitting a third data message carrying the second information bit stream to the first and second MSs by the RS.
 2. The method of claim 1, wherein the BS generates the superposition-coded data using a given coefficient for the first and second MSs by: √{square root over (1−α)}X ₁ +√{square root over (α)}X ₂, 0≦α≦1, where α is the given coefficient and X₁ and X₂ are the first and second data messages, respectively.
 3. The method of claim 1, further comprising determining transmission rates for the first and second messages using a given coefficient for the first and second MSs by: R ₁=log₂(1+(1−α)γ₀) R ₂=log₂(1+αγ_(R)), where α is the given coefficient between 0 and 1, R₁ and R₂ are the transmission rates of the messages X₁ and X₂, γ₀ is a Signal-to-Noise Ratio (SNR) of the direct link between the BS and the first MS, and γ_(R) is an SNR of a link between the BS and the RS.
 4. The method of claim 3, further comprising: calculating overall rates for all MS pairs of MSs served by the BS using the transmission rates at the scheduling instant by: $r_{1} = \frac{\log_{2}\left( {1 + {\left( {1 - \alpha} \right)\gamma_{0}}} \right)}{1 + \frac{\log_{2}\left( {1 + {\alpha\;\gamma_{R}}} \right)}{\log_{2}\left( {1 + \gamma_{2}} \right)}}$ $r_{2} = \frac{\log_{2}\left( {1 + {\alpha\;\gamma_{R}}} \right)}{1 + \frac{\log_{2}\left( {1 + {\alpha\;\gamma_{R}}} \right)}{\log_{2}\left( {1 + \gamma_{2}} \right)}}$ where r₁ and r₂ are the overall rates of two MSs of each of the user pairs; calculating a scheduling gain for the overall rates of the each user pair by a predetermined scheduling algorithm; and determining to transmit data to a user pair having a maximal scheduling gain among all user pairs.
 5. The method of claim 1, further comprising: calculating transmission rates for data messages directed to two MSs of each of all MS pairs of MSs served by the BS using a given coefficient for the each user pair; and calculating the number of bits for each of the first and second information bit streams according to the transmission rates and the number of transmission symbols used by the BS at the scheduling instant by: D₁=NR₁ D₂=NR₂ where D₁ is the number of bits for the first information bit stream, D₂ is the number of bits for the second information bit stream, N is the number of transmission symbols used by the BS, and R₁ and R₂ are the transmission rates of the data messages.
 6. The method of claim 1, further comprising determining the number of transmission symbols and a transmission rate for the third data message by: R_(s 2) = log₂(1 + γ₂) $M = \frac{D_{2}}{\log_{2}\left( {1 + \gamma_{2}} \right)}$ where R_(s2) is the transmission rate of the third data message, M is the number of transmission symbols to be used for the third data message, γ₂ is a smaller SNR between the SNRs of links between the RS and the first and second MSs, and D₂ is the number of bits for the second information bit stream.
 7. A method for receiving data via a Relay Station (RS) in a wireless relay system, comprising the steps of: receiving superposition-coded data including a first data message and a second data message by a first Mobile Station (MS) scheduled along with a second MS at a current scheduling instant by a Base Station (BS), the first data message carrying a first information bit stream for the first MS, the second data message carrying a second information bit stream for the second MS, the first MS having a relatively good direct link to the BS, and the second MS having a relatively bad direct link to the BS; receiving a third data message carrying the second information bit stream directed to the second MS from the RS connected between the BS and the first and second MSs; extracting the second information bit stream by decoding the third data message by the first MS; and acquiring the first data message by removing components related to the second information bit stream from the superposition-coded data and extracting the first information bit stream by decoding the first data message by the first MS.
 8. The method of claim 7, wherein the superposition-coded data is formed using a given coefficient for the first and second MSs by: √{square root over (1−α)}X ₁ +√{square root over (α)}X ₂, 0≦α≦1 where α is the given coefficient and X₁ and X₂ are the first and second data messages, respectively.
 9. The method of claim 7, wherein the first data message is acquired using a given coefficient for the first and second MSs, by: γ₁₂=γ₁₁ −h ₀ √{square root over (α)}X ₂ =h ₀√{square root over (1−α)}X ₁ +z ₁₁ where γ₁₂ is data including the first data message, γ₁₁ is the superposition-coded data, α is the given coefficient, X₁ and X₂ are the first and second data messages, respectively, h₀ is a channel impulse response representing channel characteristics of the direct link between the BS and the first MS, and z₁₁ is noise.
 10. The method of claim 7, wherein transmission rates of the first and second messages are determined using a given coefficient for the first and second MSs by: R ₁=log₂(1+(1−α)γ₀) R ₂=log₂(1+αγ_(R)) where α is the given coefficient between 0 and 1, R₁ and R₂ are the transmission rates of the messages X₁ and X₂, γ₀ is a Signal-to-Noise Ratio (SNR) of the direct link between the BS and the first MS, and γ_(R) is an SNR of a link between the BS and the RS.
 11. The method of claim 7, wherein the number of bits for each of the first and second information bit streams is determined according to transmission rates of the first and second data messages and the number of transmission symbols used by the BS at the scheduling instant, by: D₁=NR₁ D₂=NR₂ where D₁ is the number of bits for the first information bit stream, D₂ is the number of bits for the second information bit stream, N is the number of transmission symbols used by the BS, and R₁ and R₂ are the transmission rates of the data messages.
 12. The method of claim 7, wherein the number of transmission symbols and a transmission rate for the third data message are determined by: R_(s 2) = log₂(1 + γ₂) $M = \frac{D_{2}}{\log_{2}\left( {1 + \gamma_{2}} \right)}$ where R_(s2) is the transmission rate of the third data message, M is the number of transmission symbols to be used for the third data message, γ₂ is a smaller SNR between the SNRs of links between the RS and the first and second MSs, and D₂ is the number of bits for the second information bit stream.
 13. The method of claim 7, further comprising receiving the third data message and extracting the second information bit stream by decoding the third data message by the second MS.
 14. An apparatus for transmitting and receiving data via a Relay Station (RS) in a wireless relay system, comprising: a Base Station (BS) for encoding a first data message and a second data message directed to a first Mobile Station (MS) and a second MS, scheduled at a current scheduling instant by superposition coding, the first data message carrying a first information bit stream for the first MS, the second data message carrying a second information bit stream for the second MS, and the second MS having a relatively bad direct link to the BS, and transmitting the superposition-coded data; and the RS connected between the BS and the first and second MSs, for establishing multi-hop links, receiving the superposition-coded data from the BS, extracting the second information bit stream for the second MS by decoding the superposition-coded data, and transmitting a third data message carrying the second information bit stream to the first and second MSs, wherein the first MS receives the superposition-coded data from the BS, receiving the third data message from the RS, extracting the second information bit stream by decoding the third data message, acquiring the first data message by removing components related to the second information bit stream from the superposition-coded data, and extracting the first information bit stream by decoding the first data message.
 15. The apparatus of claim 14, wherein the superposition-coded data is formed using a given coefficient for the first and second MSs by: √{square root over (1−α)}X ₁+√{square root over (α)}X ₂, 0≦α≦1 where α is the given coefficient and X₁ and X₂ are the first and second data messages, respectively.
 16. The apparatus of claim 14, wherein transmission rates of the first and second messages are determined using a given coefficient for the first and second MSs by: R ₁=log₂(1+(1−α)γ₀) R ₂=log₂(1+αγ_(R)) where α is the given coefficient between 0 and 1, R₁ and R₂ are the transmission rates of the messages X₁ and X₂, γ₀ is a Signal-to-Noise Ratio (SNR) of the direct link between the BS and the first MS, and γ_(R) is an SNR of a link between the BS and the RS.
 17. The apparatus of claim 16, wherein the BS calculates overall rates for all MS pairs of MSs served by the BS using the transmission rates at the scheduling instant by: $r_{1} = \frac{\log_{2}\left( {1 + {\left( {1 - \alpha} \right)\gamma_{0}}} \right)}{1 + \frac{\log_{2}\left( {1 + {\alpha\;\gamma_{R}}} \right)}{\log_{2}\left( {1 + \gamma_{2}} \right)}}$ $r_{2} = \frac{\log_{2}\left( {1 + {\alpha\;\gamma_{R}}} \right)}{1 + \frac{\log_{2}\left( {1 + {\alpha\;\gamma_{R}}} \right)}{\log_{2}\left( {1 + \gamma_{2}} \right)}}$ where r₁ and r₂ are the overall rates of two MSs of each of the user pairs, calculates a scheduling gain for the overall rates of the each user pair by a predetermined scheduling algorithm, and determines to transmit data to a user pair having a maximal scheduling gain among all user pairs.
 18. The apparatus of claim 14, wherein the number of bits for each of the first and second information bit streams are calculated according to the transmission rates and the number of transmission symbols used by the BS at the scheduling instant by: D₁=NR₁ D₂=NR₂ where D₁ is the number of bits for the first information bit stream, D₂ is the number of bits for the second information bit stream, N is the number of transmission symbols used by the BS, and R₁ and R₂ are the transmission rates of the data messages.
 19. The apparatus of claim 14, wherein the number of transmission symbols and a transmission rate for the third data message are determined by: R_(s 2) = log₂(1 + γ₂) $M = \frac{D_{2}}{\log_{2}\left( {1 + \gamma_{2}} \right)}$ where R_(s2) is the transmission rate of the third data message, M is the number of transmission symbols to be used for the third data message, γ₂ is a smaller SNR between the SNRs of links between the RS and the first and second MSs, and D₂ is the number of bits for the second information bit stream.
 20. The apparatus of claim 14, wherein the first MS acquires the first data message using a given coefficient for the first and second MSs, by: γ₁₂=γ₁₁ −h ₀ √{square root over (α)}X ₂ =h ₀√{square root over (1−α)}X ₁ +z ₁₁ where γ₁₂ is data including the first data message, γ₁₁ is the superposition-coded data, α is the given coefficient, X₁ and X₂ are the first and second data messages, respectively, h₀ is a channel impulse response representing channel characteristics of the direct link between the BS and the first MS, and z₁₁ is noise.
 21. The apparatus of claim 14, wherein the second MS receives the third data message and extracting the second information bit stream by decoding the third data message. 